Random access areas are provided in mobile communication systems represented by cellular communication systems and wireless LAN (Local Area Network) systems. A random access area refers to an uplink transmission area that is provided as when a communication terminal requests an initial access to a base station (or an access point) or when a new resource allocation request is issued in a centralized management system where a base station allocates transmission time and transmission band to communication terminals.
The transmission timing for a random access channel (RACH) transmitting in a random access area is normally determined based on the synchronization timing in downlink. That is, a communication terminal establishes downlink frame timing synchronization using, for example, a frame synchronization pilot signal transmitted from a base station in downlink, and transmits RACH to a designated random access area.
Accordingly, when the distance between the base station and the communication terminal is longer, the time a RACH transmitted from the communication terminal takes to arrive at the base station increases. That is to say, if the time a downlink frame synchronization signal arrives at a communication terminal is delayed due to propagation delay, the frame timing at the communication terminal is also delayed by the length of the propagation delay. Moreover, this communication terminal starts transmitting the RACH based on its own frame timing, and, consequently, the arrival of the RACH is further delayed by the length of propagation delay. As a result, the arrival time for the RACH is delayed twice as much as a case without propagation delays.
Non-patent document 1 discloses a technique of assigning a guard time equal to or greater than the maximum propagation delay anticipated in the system between a RACH frame (the frame where RACH is allocated) and a frame of the subsequent another channel, to reduce or prevent inter frame interference occurred by overlapping the tail of RACH of large propagation delay and the beginning of the subsequent channel.
For example, as shown in FIG. 1, the RACH frame is formed with a preamble part (a pilot signal) and a data part (terminal ID information, access request information/resource allocation request information and others). In an random access area, as shown in FIGS. 2A and 2B, RACH frame length and guard time are provided such that the tail of the RACH from the communication terminal (UE3 in the figure) located at the farthest position from the base station stays within the random access area. This is to prevent the tail of the RACH frame from entering the next area and producing interference between the RACH frame and the frame transmitted in the next transmission area.    Non-patent Document 1: Ericsson, “E-UTRA Random Access”, R1-051445, 3GPP TSG RAN WG1 Meeting #43, Seoul, Korea, Nov. 7-11, 2005.